1. Field of the Invention
The present invention relates to methods for detecting bubbles in electrochemical cells, a related apparatus and a method for retrofitting existing electrochemical cells with a bubble detection apparatus.
2. Description of the Related Art
A typical micro fuel cell oxygen sensor, also known as a galvanic electrochemical oxygen sensor, includes a cathode and an anode sealed in a housing (sensor body) filled with an appropriate electrolyte. Oxygen diffuses into the sensor body through a thin sensing membrane or electrode. Sensors of this type typically are sealed and are disposable. The sensors typically include: a lead anode, a noble metal cathode (gold or silver), an aqueous electrolyte solution, such as potassium hydroxide (KOH), a plastic body (container) with electrical contacts between the cathode and the anode and a diffusion barrier of Teflon or other materials as are known in the art.
Electrochemical oxygen sensors, as well any as other gas sensors and galvanic cells in which a liquid electrolyte is used, are susceptible to failure due to water loss which results in the formation of a gas pocket between the primary anode and cathode. In the case of oxygen sensors, as is the case with many galvanic cells, failure of the cell due to the formation of a gas pocket between the electrodes is expected, but the exact time of failure typically cannot be anticipated. In industrial systems, to replace a sensor a manufacturing process often must be shut down. The failure of an oxygen sensor, or other galvanic cell-type sensors or devices, causes costly and unpredictable operational shutdowns. Thus, a system is desired that automatically detects the formation of gas pockets in galvanic cells and sensors, such as electrochemical oxygen sensors, and therefore advises of imminent cell or sensor failure.
Provided herein is a method for detecting the presence of a gas pocket in an electrochemical cell, such as an oxygen sensor. The method may be used to predict imminent failure of the cell. In one embodiment, a light beam is directed through electrolyte in the cell between electrodes at an angle so that the light beam is refracted when no bubble is present in the electrolyte and is refracted to a lesser degree, or is not refracted, when a gas pocket is present in the path of the light beam.
In a second embodiment, a sensor electrode is placed in an area of the galvanic cell in which a gas pocket collects prior to failure of the cell. Imminent cell failure is detected by loss of electrical contact, due to loss of intervening electrolyte between the sensor electrode and a counter-electrode. The counter-electrode may be a primary electrode of the electrochemical cell.
Thus, the present invention is generally directed to an electrochemical cell assembly. The assembly includes an electrochemical cell that includes a container (sensor housing) having a wall defining an interior volume and typically, end portions, the cell includes a primary anode, a primary cathode and an electrolyte. The electrochemical cell assembly also includes a gas pocket detection assembly suitably configured to detect formation and/or enlargement of a gas pocket in the container. The detection assembly detects the gas pocket prior to failure of the electrochemical cell from loss of electrolyte contact (continuity) between the primary anode and the primary cathode resulting from further enlargement of the gas pocket.
In an electrical bubble detection embodiment within the present invention, the gas pocket detector assembly includes a secondary electrode suitably configured within the container to detect loss of electrolyte contact (continuity) between the secondary electrode and a detector counter-electrode prior to failure of the electrochemical cell due to loss of electrolyte contact between the primary anode and the primary cathode resulting from further enlargement of the gas pocket. In the embodiment, the detector counter-electrode may be a primary electrode.
In an optical bubble detection embodiment within the present invention, the wall of the container comprises a first clear portion and a second clear portion suitably configured so that a light beam can pass through the container. The gas pocket detector assembly includes a light beam source adjacent to the first clear portion. The light beam source is configured to direct a light beam on a path through the first clear portion and the second clear portion of the container. A light beam detector is configured to detect the light beam exiting from the container through the second clear portion. The detector is selected and/or configured to produce a first signal when the light beam travels a direct path through the interior of the container (when no gas pocket in the electrolyte is located in the path of the light beam) and second signal when the light beam travels a refracted path through the interior of the container (when a gas pocket is present in the electrolyte in the path of the light beam). The first signal and the second signal may be one of a signal generated when a light beam strikes a detector, or the lack of a signal when no light beam strikes the detector.
The present invention also provides a method for predicting failure of an electrochemical cell as described above. The method generally includes the step of providing a gas pocket detector assembly to detect formation of or enlargement of a gas pocket in the container prior to failure of the electrochemical cell due to loss of electrolyte contact between the anode and the cathode resulting from formation or enlargement of the gas pocket. In a second step of the method, the gas pocket detector assembly is used to monitor the cell for the formation or enlargement of a gas pocket in the container. The gas pocket detector assembly may be an electrical or optical detector, as described above.
In another embodiment, a method is provided for retrofitting an electrochemical gas sensor device including an electrochemical cell, as described above. The method includes the step of attaching a gas pocket detection assembly to the gas sensor device. The gas pocket detection assembly is suitably configured to detect formation or enlargement of a gas pocket in the container prior to failure of the electrochemical cell due to loss of electrolyte contact between the anode and the cathode resulting from further enlargement of the gas pocket.
Also provided is a kit for retrofitting an electrochemical gas sensor device. The kit may include a gas pocket detector assembly that is suitably configured to attach to a gas sensor device to detect formation or enlargement of a gas pocket in an electrochemical cell in the gas sensor device. The kit may include a bracket or brackets configured to engage a structure in the gas sensor device other than the electrochemical cell. The bracket or brackets position a light beam source so that the light beam source directs a light beam on a path through the interior volume of the container and/or position a light beam detector to detect the light beam exiting from the container. The detector may be selected and/or configured to produce a first signal when the light beam travels a direct path through the interior volume of the container and/or a second signal when the light beam travels a refracted path through the interior volume of the container.